Ulysses Reveals Global Solar Wind Plasma Output at 50-Year Low

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DC Agle 818-393-9011
Jet Propulsion Laboratory, Pasadena, Calif.
agle@jpl.nasa.gov

Dwayne Brown 202-358-1726
NASA Headquarters, Washington
dwayne.c.brown@nasa.gov

NEWS RELEASE: 2008-178 September 23, 2008

Ulysses Reveals Global Solar Wind Plasma Output at 50-Year Low

PASADENA, Calif. -- Data from the Ulysses spacecraft, a joint NASA-European Space
Agency mission, show the sun has reduced its output of solar wind to the lowest levels
since accurate readings became available. The sun's current state could reduce the natural
shielding that envelops our solar system.

"The sun's million mile-per-hour solar wind inflates a protective bubble, or heliosphere,
around the solar system. It influences how things work here on Earth and even out at the
boundary of our solar system where it meets the galaxy," said Dave McComas, Ulysses'
solar wind instrument principal investigator and senior executive director at the
Southwest Research Institute in San Antonio, Texas. "Ulysses data indicate the solar
wind's global pressure is the lowest we have seen since the beginning of the space age."

The sun's solar wind plasma is a stream of charged particles ejected from the sun's upper
atmosphere. The solar wind interacts with every planet in our solar system. It also defines
the border between our solar system and interstellar space.

This border, called the heliopause, is a bubble-shaped boundary surrounding our solar
system where the solar wind's strength is no longer great enough to push back the wind of
other stars. The region around the heliopause also acts as a shield for our solar system,
warding off a significant portion of the cosmic rays outside the galaxy.

"Galactic cosmic rays carry with them radiation from other parts of our galaxy," said Ed
Smith, NASA's Ulysses project scientist at the Jet Propulsion Laboratory in Pasadena,
Calif. "With the solar wind at an all-time low, there is an excellent chance the heliosphere
will diminish in size and strength. If that occurs, more galactic cosmic rays will make it
into the inner part of our solar system."

Galactic cosmic rays are of great interest to NASA. Cosmic rays are linked to
engineering decisions for unmanned interplanetary spacecraft and exposure limits for
astronauts traveling beyond low-Earth orbit.

In 2007, Ulysses made its third rapid scan of the solar wind and magnetic field from the
sun's south to north pole. When the results were compared with observations from the
previous solar cycle, the strength of the solar wind pressure and the magnetic field
embedded in the solar wind were found to have decreased by 20 percent. The field
strength near the spacecraft has decreased by 36 percent.

"The sun cycles between periods of great activity and lesser activity," Smith said. "Right
now, we are in a period of minimal activity that has stretched on longer than anyone
anticipated."

Ulysses was the first mission to survey the space environment over the sun's poles. Data
Ulysses has returned have forever changed the way scientists view our star and its effects.
The venerable spacecraft has lasted more than 17 years, or almost four times its expected
mission lifetime. The Ulysses solar wind findings were published in a recent
edition of Geophysical Research Letters.

The Ulysses spacecraft was carried into Earth orbit aboard space shuttle Discovery on
Oct. 6, 1990. From Earth orbit it was propelled toward Jupiter, passing the planet on Feb.
8, 1992. Jupiter's immense gravity bent the spacecraft's flight path downward and away
from the plane of the planets' orbits. This placed Ulysses into a final orbit around the
sun that would take it over its north and south poles.

The Ulysses spacecraft was provided by ESA, having been built by Astrium GmbH
(formerly Dornier Systems) of Friedrichshafen, Germany. NASA provided the launch
vehicle and the upper stage boosters. The U.S. Department of Energy supplied a
radioisotope thermoelectric generator to power the spacecraft. Science instruments were
provided by U.S. and European investigators. The spacecraft is operated from JPL by a
joint NASA-ESA team.

More information about the Ulysses mission is available at http://ulysses.jpl.nasa.gov .

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